1903 Won't Respond to Tip-In Procedure

[bry593]

Since my last post, I've wired the secondaries shut and have been driving it back and forth to work.  No issues although it has a little lag, under part throttle with the choke still on.  Once warm, runs like a 2-barrel.  No sag or surge, but noticeably less power when pulling into traffic.  I have also tried WOT and found nothing out of the ordinary.

Having the 2ndys wired shut lowered warm idle a few hundred rpm.  I assume this is because the wire is holding the throttle plates more closely shut.  This results in less air getting into the engine.

As for detonation, there is some slight ping from the vacuum advance while in transition mode.  This morning, I plugged the Vac advance and will run some additional tests to see if the ping is solely due to vac advance.

Ordered some colder spark plugs.  Changing from R44LTS, to MR43LTS.  The "M" means marine, so has a stainless steel body.  Will see what affect these have on ping.  If not enough, might retard the timing back to 8 degrees BTDC. 

Here are the rods and jets I am considering and % fuel change relative to current setup:
Rod      Jet    Cruise   WOT     C Area  WOT Area   Cruise %  WOT %
50M   0.073   0.050   0.036   0.0022   0.0032     -4.94%   -3.52%   <<Stock 1903
50C   0.074   0.050   0.036   0.0023   0.0033      0.00%   0.00%    <<Current
48K   0.073   0.048   0.026   0.0024   0.0037      1.65%   11.32%   <<.006 Taper, Step to .026
45C   0.072   0.045   0.026   0.0025   0.0035      6.15%   7.85%     <<Single Taper
45C   0.073   0.045   0.026   0.0026   0.0037      11.02% 11.32%   
50P   0.074   0.050   0.026   0.0023   0.0038      0.00%   14.83%    <<.012 Taper, Step to .026

[bry593]

LOL, figured out why my idle was a couple hundred rpm lower.  Remember when I said I disconnected the vacuum advance this morning?  Ha, I'm an idiot.... 

I plan to modify the advance stop window so there is absolutely no advance until 10 in Hg.  I can do that by applying the mighty vac, pumping it to 10 in Hg, and then welding a sheet metal stop over the gap between the pull rod and end of window.  I might even see if I can lengthen the other end of the window so it stops at 2 in Hg below my idle vacuum (19.5 - 2 = 17.5 or 16-17ish).  Hopefully there is enough stroke to accommodate moving the window stop.  Seems like this should work.  Will let you know how it turns out.

I think most HEI advances are made to work with smogged, detuned engines of the 70's and 80's.  That's why I can't find anything that actually matches my ideal: starts @ 10, and all in at 17, with 18-22 degrees at the crank.

[Pav8427]

FWIW. Do a search for Lars Grimsrud HEI timing papers. He has a pretty extensive list of factory VA's.
Might be something in there that suits your needs.

[bry593]

Thanks PAV, that is good advice.  I didn't want to bring it up on Cliff's forum, but yes, Lars has great information on tuning the Quad and distributor. 

At this point, I've pretty much read everything on the net:  Lars, Cliff, Wallace Racing, Tuner @ Racing Fuel Systems, etc.  All are pretty sharp guys, but do have varying viewpoints.  Lars doesn't discuss calibration of the idle circuit, Tuner likes the idle circuit lean, Cliff likes rich since rich runs, Wallace must be an engineer 'cause he has a calculator for everything.... 

I'm stubborn as hell, and have to learn things by doing.  So I take everyone's advice and then do what I want.  Win some, lose some, but always gain experience.  I'll tell you, I'm almost to the point where I will follow Cliff's advice to drop in the ZZ4 cam... almost.....

[Kenth]

I have found chasing a "magic" timing curve to ignite a too lean A/F mixture is a waste of time and leaves you endlessly chasing your tail, and an engine that will never run right or at it´s full potential.

Been there done that.

Nowadays i mostly use the proper factory timing curve, or very close to it, per engine application and add proper amounts of fuel into the mixture were needed, idle, off-idle, main and wot circuits, the engine will tell you were the need is.
Hesitation is an indicator of a too lean A/F mixture, and so is pinging.

Hats off for Cliff sharing his hands on experience with these things!

[Cliff Ruggles]

Well said Kenth, we actually posted at exactly the same time so below I just left my comments on what you were trying to convey. 

Cliff doesn't like "rich", I tune every carb and distributor exactly for the application.

As for predicting fuel delivery with metering area, well, sadly it just doesn't work that way. 

Engine load, vacuum, airflow thru the carb/across the venturi areas, pressure differential above and below the throttle plates, fuel added by the transfer slot, idle mixture holes and LIAB plus the size of the idle and MAB's all have an impact on fuel delivery across the load/speed range.

I've NEVER once found success trying to use stronger distributor springs or VA cans with stronger springs in them (delay timing or have it fall out sooner) tuning one of these engines.  Superficially yes, but in the long haul other changes were needed to get the combo up to par and the most out of it.

The problem here is fundamental, too much cylinder pressure for the octane of fuel being used.  So you have to retard the total timing (mechanical) and limit the amount of vacuum advance added until it stops pinging.  A/F is such a small player there not really worth mentioning as we NEVER flood an engine with fuel to cure a detonation issue, that just leads to heavy fuel consumption and in most cases it pings anyhow.

Playing around with distributor springs and VA cans is more of a "crutch" fix than to cure the issues and get the best end results.

A cam change will be the best move, and you will also be nicely rewarded with a LOT more power with the right camshaft.  A small cam gets you in several ways, it does too good a job of cylinder filling early and with a tight LSA version you get a high spike in cylinder pressure early in the RPM range.  A small cam in a large engine KILLS power clear across the RPM range especially when you have to start pulling out timing to make things happy.

Change the cam, bigger and wider LSA and your tuning efforts will be nicely rewarded with a BUTTLOAD more power over a broader RPM range, and no detonation anyplace.......FWIW.....Cliff

[bry593]

Is there a specific cam you would recommend for this application?  As I mentioned, I do have a ZZ4 cam.  And I can get creative with the springs and retainers to permit greater than .470 lift.  Does Melling or someplace like that make an "RV" roller cam?  That is basically what I need.  Would be great if I don't have to mess with the valve springs and retainers.

How about this one:  https://www.summitracing.com/parts/mel-22129/make/chevrolet

[Cliff Ruggles]

I'm not even sure what's out there for hydraulic roller cams these days.  I got out of the engine building business about 4 years ago.  I've had very good success with factory roller cams dating clear back to when they started making roller blocks.  I've never built an older Chevy with a roller in it, only the 1987 and newer blocks.

Going from memory some of the late 80's/early 90 TBI engines had some really nice cams in them on wider LSA's.  There were pretty "low" lift too.  I used one once in a 350 truck engine with early Vortec heads and it was an absolute HOME RUN in that build.  I also used flat top pistons and thin head gasket, so the compression was up there some.  The 350 went into my dads 1991 3/4 ton truck backed by a NV4500 and 3.73 gears.  It pulled like a big block and got excellent fuel economy when he wasn't towing with it.  I'm sure they make something these days along those lines.  They didn't have a lot of lift and were easy on the valve train as well.

I'd recommend a cam with about 10 degrees more duration than you have now out on a 112.  I hate the word "RV" but a 204/214 @ .050" cam would work very well in the 383 with 9 to 1 compression.......CLiff

[bry593]

I took Kenth's suggestion and put it to GM factory specs.  That means I've been driving around with the vacuum advance unplugged, only I did not install a 750 Holley.   

I swapped out to colder plugs R43LTS ->  Still have detonation at WOT
I dropped static timing to 8 degrees -> Still have detonation at WOT

So, per Cliff's advice, have sought out a cam that does not suck.  Called Howard's Cams and they recommended a 180225-12:  .450/.465 207/213@.050 112 separation, idle - 4200.  It should be about as good as it gets when used with factory Vortec springs and retainers.

My initial tune will be similar to Cliff's original recommendation.  Hopefully reduced dynamic compression will permit the engine to use 87 pure gas, without WOT ping and engine run-on:

IT .036
UIAB (horn) .050 (1.25mm)
LIAB .064
50C needles
74 jets

I'm feeling optimistic about this......

[tayto]

cliff gave me some guide lines to help select a cam for my "vortech" heads. these are the TBI heads that GM used from 87-95 before the "vortec" heads came out. Most consider them door stops but I did some pocket porting on them and put them on my 355. Flat tops with a tight 0.034" quench, i think compression worked out to 10.3:1. Cam was also a Howards cam 213/217 @ 0.050" 112LSA 0.485"/0.495" lift. Built motor for towing, seems to run fine on 91,haven't towed anything yet though.

[Cliff Ruggles]

I actually like the early swirl port "Vortec" heads for a truck/towing engine.  They bring the power in early and didn't fair two bad anyplace in your 355 by looking at the dyno sheet.....

[bry593]

Yes, that is impressive for TBI swirl heads.

Here is a useful reference from the Crankshaft Coalition discussing Cam Duration vs Compression:
https://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

[Cliff Ruggles]

Good info.  I especially like this part:

"The chart will show why it is foolish to install a 230 degree cam in an 8.00:1 motor in an attempt to get a lope out of the motor. It might lope, but it will lack the power to pull the hat off your head. There seems to be an obsession with lope these days. Lope is simply the sound of the motor being inefficient at low rpm's because the ascending piston is pushing fuel/air mixture back up the intake tract through the still open intake valve and disrupting the metering abilities of the carburetor."

Although they didn't take CID into consideration the info is still valid and the chart reflects what you'd see from a 350cid.  A 454 on the other hand will gobble up a 230 @ .050" cam and run very well with it, especially if it had moderate compression.  Below is a dyno chart from a 455 Pontiac Super Duty build around 8.8 to 1 compression.  The cam was a 230/236/112LSA HR with 281/287 @ .006" duration.  It idled dead smooth making 16" vacuum at 650rpm's with only 10 degrees timing in it.

Put the same cam in a 350 engine with under 9 to 1 compression and I'll bet you'd be lucky to see 8-9" vacuum without running the initial timing clear off the scale at idle speed!......

[tayto]

cliff, bry: i wish I would have known the dyno graph went lower than 3000. i suspect the peak torque is higher and a bit lower down in the rpm range. also, it made the most power at 26*. i had to persuade the owner/dyno operator to dial back the timing from 32*. he did so 2* at a time made about 10hp/15tq more peal power with 6* less timing!

EDIT: I currently have 3.42s with 30" tire, this thing needa more gear, I think i will goto 4.11s which will help a lot for towing IMO

[Cliff Ruggles]

The better job that you do with the engine build the LESS timing and fuel is needed to make highest peak power.  Those early swirl intake port heads must do a great job at cylinder filling early in the RPM range so it's now wanting a lot of timing and making a lot of power early. 

Most of the engines I've done here in the last 25 years have made best power with around 28-32 degrees and another 10-14 from the vacuum advance.

My own engine runs equally as well at 28 vs 30 or 32 degrees at the track, so I run it with 28 degrees......Cliff